Method of Manufacturing a Logging Tool for a Borehole

ABSTRACT

Methods of manufacturing a logging tool for a borehole are provided. In an embodiment of the present invention, shields are disposed to prevent or reduce circulating electrical currents from causing a voltage difference that would adversely affect the measured voltage difference between two adjacent electrodes. Groups of sensors, e.g., two or more, may be placed within the shield. The shields may have any cross-section shape. A magnetic field source is disposed proximate the electrodes such that the magnetic field source is oriented to generate a magnetic field substantially orthogonal to an imaginary line intersecting each pair of electrodes. A voltage measuring circuit is electrically coupled to the plurality of electrodes and configured to measure a voltage difference between each pair of electrodes.

This application is a divisional of U.S. patent application Ser. No.13/681,047 filed on Nov. 19, 2012, now U.S. Pat. No. 8,875,379, whichapplication is a continuation of U.S. patent application Ser. No.12/987,773, filed on Jan. 10, 2011, now U.S. Pat. No. 8,312,767, whichapplication is a continuation of U.S. patent application Ser. No.12/497,934, filed on Jul. 6, 2009, now U.S. Pat. No. 7,866,207, whichapplication is a continuation of U.S. patent application Ser. No.10/574,330, filed on Mar. 31, 2006, now U.S. Pat. No. 7,574,907, whichapplication is a national filing under 35 U.S.C. §371 of InternationalApplication No. PCT/US2004/032129, filed on Sep. 30, 2004, whichapplication claims the benefit of U.S. Provisional Application No.60/507,720, filed on Oct. 1, 2003, entitled “Apparatus and Method forFluid Flow Measurement with Sensor Shielding,” all of which applicationsare hereby incorporated herein by reference.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to the following co-pending andcommonly-assigned patent applications: application Ser. No. 13/561,973,filed Jul. 30, 2012, entitled “Fluid Flow Measuring Device and Method;”and application Ser. No. 13/447,962, filed Apr. 16, 2012, entitled“Rotating Fluid Flow Measurement Device and Method.”

TECHNICAL FIELD

This invention relates generally to fluid flow sensing devices andmethods and, more particularly, to fluid flow sensing devices andmethods utilizing sensor shielding.

BACKGROUND

An oil and gas well is shown in FIG. 1 generally at 60. Wellconstruction involves drilling a hole or borehole 62 in the surface 64of land or ocean floor. The borehole 62 may be several thousand feetdeep, and drilling is continued until the desired depth is reached.Fluids such as oil, gas and water reside in porous rock formations 68. Acasing 72 is normally lowered into the borehole 62. The region betweenthe casing 72 and rock formation 68 is filled with cement 70 to providea hydraulic seal. Usually, tubing 74 is inserted into the hole 62, thetubing 74 including a packer 76 which comprises a seal. A packer fluid78 is disposed between the casing 72 and tubing 74 annular region.Perforations 80 may be located in the casing 72 and cement 70, into therock 68, as shown.

Production logging involves obtaining logging information about anactive oil, gas or water-injection well while the well is flowing. Alogging tool instrument package comprising sensors is lowered into awell, the well is flowed and measurements are taken. Production loggingis generally considered the best method of determining actual downholeflow. A well log, a collection of data from measurements made in a well,is generated and is usually presented in a long strip chart paper formatthat may be in a format specified by the American Petroleum Institute(API), for example.

The general objective of production logging is to provide informationfor the diagnosis of a well. A wide variety of information is obtainableby production logging, including determining water entry location, flowprofile, off depth perforations, gas influx locations, oil influxlocations, non-performing perforations, thief zone stealing production,casing leaks, crossflow, flow behind casing, verification of new wellflow integrity, and floodwater breakthrough, as examples. The benefitsof production logging include increased hydrocarbon production,decreased water production, detection of mechanical problems and welldamage, identification of unproductive intervals for remedial action,testing reservoir models, evaluation of drilling or completioneffectiveness, monitoring Enhanced Oil Recovery (EOR) process, andincreased profits, for example. An expert generally performsinterpretation of the logging results.

In current practice, measurements are typically made in the centralportion of the wellbore cross-section, such as of spinner rotation rate,fluid density and dielectric constant of the fluid mixture. These datamay be interpreted in an attempt to determine the flow rate at any pointalong the borehole. Influx or exit rate over any interval is thendetermined by subtracting the flow rates at the two ends of theinterval.

In most producing oil and gas wells, the wellbore itself generallycontains a large volume percentage or fraction of water, but oftenlittle of this water flows to the surface. The water that does flow tothe surface enters the wellbore, which usually already contains a largeamount of water. The presence of water already in the wellbore, however,makes detection of the additional water entering the wellbore difficultand often beyond the ability of conventional production logging tools.

Furthermore, in deviated and horizontal wells with multiphase flow, andalso in some vertical wells, conventional production logging methods arefrequently misleading due to complex and varying flow regimes orpatterns that cause misleading and non-representative readings.Generally, prior art production logging is performed in these complexflow regimes in the central area of the borehole and yields frequentlymisleading results, or may possess other severe limitations. Often thelocation of an influx of water, which is usually the information desiredfrom production logging, is not discernable due to the small change incurrent measurement responses superimposed upon large variations causedby the multiphase flow conditions.

U.S. patent application Ser. No. 09/880,402, filed Jun. 13, 2001,entitled “Conductive Fluid Logging Sensor and Method,” now issued asU.S. Pat. No. 6,711,947 B2, Ser. No. 10/600,053, filed Jun. 20, 2003,entitled “Conductive Fluid Logging Sensor and Method,” now issued asU.S. Pat. No. 6,799,407 B2, and Ser. No. 10/924,320, filed Aug. 23,2004, entitled “Fluid Flow Measuring Device and Method of ManufacturingThereof,” now issued as U.S. Pat. No. 6,971,271 B2, disclose apparatusesand methods for measuring the flow of fluid as it enters or exits aninterior wall of a fluid conduit before the fluid becomes substantiallyintermixed with the fluids and the often complex flow pattern already inthe fluid conduit. In particular, the apparatuses and methods may beused to measure the radial flow of conductive fluid through the wall ofa fluid conduit, generally without being sensitive to non-conductivefluid flow or to non-radial conductive fluid flow. As an applicationexample, embodiments may be used to detect and measure the radial flowof water through the conduit or borehole wall of an oil or gas well.

As described in the above-referenced patent applications, an electricfield is induced when water or generally any material moves through amagnetic field. When the material has at least a small amount ofconductivity, the voltage difference generated by the induced electricfield between two points may be measured. Generally, the voltage fromthe induced electric field is proportional to the velocity of the fluidmedium. If this voltage is measured, the velocity of the medium may bedetermined.

In some cases, however, another effect may also contribute to themeasured voltage. In particular, a voltage drop due to the circulationof electrical currents in the fluid may introduce an extra voltagecomponent into the measured voltage. These circulating electricalcurrents generally may be caused by the ubiquitous induced electricfields in the vicinity of the two electrodes, and may depend upon thevelocity distribution of the fluid, as well as the values and locationsof all the induced electric fields in the proximity of the measureelectrodes.

Generally, the voltage difference in a localized area (such as betweentwo closely spaced electrodes) contributed by these circulating currentsis dependent upon the conductivity of the medium and the localizedcurrent density. In addition, the value of the circulating current maychange from point to point. Thus, under some circumstances or in someapplications, the measured voltage difference between two electrodes mayintroduce an error component and interfere with a reasonably accuratefluid velocity measurement.

SUMMARY OF THE INVENTION

These and other problems are generally solved or circumvented, andtechnical advantages are generally achieved, by preferred embodiments ofthe present invention that utilize shields between electrode pairs.Generally, shields substantially reduce or prevent circulating currentsfrom contributing an undesired component to the measured voltage, thushelping to ensure that the voltage measurement provides a voltage valuethat is proportional to the velocity of the fluid flow.

In a preferred embodiment of the present invention, using shieldsbetween groups of electrodes generally inhibits or prevents circulatingelectrical currents from causing a voltage difference that wouldadversely affect the measured voltage difference between two adjacentelectrodes. Generally, once the circulating electrical current flowbetween the electrode pairs is substantially reduced or eliminated, onlythe voltage difference from the localized induced electric fieldremains. This way, a true induced voltage may be measured, and thus anaccurate value for the fluid velocity may be determined.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures or processes for carrying outthe same purposes of the present invention. It should also be realizedby those skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the invention as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a cross-section view of an oil or gas well;

FIG. 2A shows a perspective view of the sensor loop moving towards awater inflow;

FIG. 2B shows a perspective view of the sensor loop positioned over awater inflow, and thus sensing the water inflow;

FIG. 3A is a perspective view of a rectangular shield;

FIG. 3B is a perspective view of a circular shield;

FIG. 4 is a perspective view of a rectangular shield having amid-section narrower than the ends;

FIG. 5 is a perspective view of a rectangular shield having amid-section wider than the ends;

FIG. 6 is a perspective view of a circular shield having a mid-sectionnarrower than the ends;

FIG. 7 is a perspective view of a circular shield having a mid-sectionwider than the ends;

FIG. 8 is a perspective view of a rectangular shield having a taperedperimeter;

FIG. 9 is a perspective view of a circular shield having a taperedcircumference;

FIG. 10 is a perspective view of a rectangular shield having pointelectrodes;

FIG. 11 is a perspective view of a rectangular shield havingmedium-sized electrodes;

FIG. 12 is a perspective view of a rectangular shield having large-sizedelectrodes;

FIG. 13 is a perspective view of a rectangular shield wherein electrodesare mounted on opposing sides of the rectangular shield;

FIG. 14 is a perspective view of multiple rectangular shields;

FIG. 15 is a perspective view of multiple circular shields; and

FIG. 16 is a perspective view of multiple circular shields wherein thespace between the circular shields is filled.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed herein are merely illustrative of specific ways tomake and use the invention, and do not limit the scope of the invention.

The present invention will be described with respect to preferredembodiments in a specific context, namely an apparatus and method fordetecting radial water flow through the borehole wall of an oil or gaswell. The invention may also be applied, however, to other apparatusesand methods that generally would benefit from the detection ofconductive fluid flow in one direction while being insensitive tononconductive fluid flow in the same direction or to either type offluid flow in orthogonal directions. For example, the invention may beused to detect conductive fluid flow leaks in some other type of fluidconduit, such as a water pipeline, a chemical line or a sewer line.

As will be described in detail below, a wide variety of shield designsand electrode designs may be used in accordance with preferredembodiments of the present invention. All of these designs, as well asmany other variations that one of ordinary skill in the art wouldunderstand to be within the scope of the present invention, may be usedwith the apparatuses and methods disclosed in U.S. patent applicationSer. No. 09/880,402, filed Jun. 13, 2001, entitled “Conductive FluidLogging Sensor and Method,” now issued as U.S. Pat. No. 6,711,947 B2,Ser. No. 10/600,053, filed Jun. 20, 2003, entitled “Conductive FluidLogging Sensor and Method,” now issued as U.S. Pat. No. 6,799,407 B2,and Ser. No. 10/924,320, filed Aug. 23, 2004, entitled “Fluid FlowMeasuring Device and Method of Manufacturing Thereof,” now issued asU.S. Pat. No. 6,971,271 B2. For example, the shield and electrodedesigns disclosed herein may be used with the devices disclosed in FIGS.2-10 of the above-mentioned patent applications.

FIG. 2A schematically illustrates a perspective view of a sensor loop108 positioned against the inside of the casing 111 b wall as describedin U.S. patent application Ser. No. 09/880,402, filed Jun. 13, 2001,entitled “Conductive Fluid Logging Sensor and Method,” now issued asU.S. Pat. No. 6,711,947 B2, Ser. No. 10/600,053, filed Jun. 20, 2003,entitled “Conductive Fluid Logging Sensor and Method,” now issued asU.S. Pat. No. 6,799,407 B2, and Ser. No. 10/924,320, filed Aug. 23,2004, entitled “Fluid Flow Measuring Device and Method of ManufacturingThereof,” now issued as U.S. Pat. No. 6,971,271 B2, which areincorporated herein by reference, having shields 120 interposed betweenadjacent pairs of sensors. Generally, the sensor loop 108 is adapted tomove upward and downward over perforation holes 112 through the casing111 b towards an inflow of water 113 through perforation holes 112.

In an oil or gas well, water inflow is undesirable, therefore thelocation of the water inflow is important information to obtain so thatthe casing 111 b can be repaired, for example. No inflow of water ismeasured in the sensor loop 108 position shown in FIG. 2A, because thereis no inflow of water 113 anywhere over the sensor loop 108. Preferably,the sensor loop 108 remains flush with the casing 111 b to maintainclose proximity to regions of inflow of water 113, in order to directlysense the inflow of water 113. FIG. 2B shows the sensor loop 108 againstthe inside of the casing wall 111 b moving downward and actually at thelocation of the inflow of water 113 through a perforation hole 112. Inthis position the sensor loop 108 detects the inflow of water 113.

In accordance with an embodiment of the present invention, one or moreshields 120 are positioned between groups of electrodes in a sensorloop, wherein each group of electrodes may have two or more electrodes.(Pairs of electrodes are illustrated herein for illustrative purposesonly.) While the shape and size of the shields 120 may vary, it shouldbe noted that at least a portion of each electrode should be exposed tothe fluid to allow detection of conductive fluid flow. The position ofthe electrode with respect to the shield may be adjusted to maximize thedetection of conductive fluid flow.

The shields 120 may be formed of a conducting material (e.g., a metal)or an insulating material (e.g., a dielectric). Use of an insulatingmaterial, such as a plastic, however, is generally preferable.

The shields 120 may be of any design that adequately prevents or reducesthe circulating electrical currents from interfering with measurement ofthe voltage difference caused by a conductive fluid flow inducedelectric field. It should be noted, however, that the shields 120 areillustrated in FIGS. 2 a and 2 b as single planar dividers interposedbetween pairs of electrodes for illustrative purposes only to illustratethe desired isolation effect between groups of electrodes. Preferably,the shields 120 form a conduit through which the conductive fluid flowpasses. The shields 120 may, for example, be tubes having circular,square, rectangular, triangular, elliptical, hexagonal, octagonal,irregular-shaped, a combination of any of the above, or the likecross-section. Generally, any shape shield may be used that allows fluidflow in the desired direction while substantially inhibiting circulatingelectrical currents from interfering with the electrodes.

FIGS. 3 a and 3 b are perspective views of shields having a squarecross-section and a circular cross-section, respectively, in accordancewith an embodiment of the present invention. Referring first to FIG. 3a, a square shield 310 is illustrated. The square shield 310 preferablyhas solid surfaces 312 and open ends 314, thereby allowing fluids toflow through the square shield 310 as indicated by directional arrows316. Similarly, FIG. 3 b illustrates a cylindrical shield 320 having asolid side surface 322 and open ends 324. Other shapes may be used.

The shields, such as those illustrated in FIGS. 3 a and 3 b, arepreferably positioned such that the flow of fluid flowing through theshields is substantially parallel to the flow of conductive fluid thatis to be detected. In the embodiment of the borehole discussed above,the shields are preferably positioned such that the flow of fluidthrough the shields is substantially orthogonal to the longitudinal axisof the borehole or conduit in order to detect the radial flow ofconductive fluid through the wall of the borehole or conduit. In thismanner, the interference from the induced electric field due to the flowof conductive fluid is limited and limits the interference to adjacentsensors.

Additional variations in the shield design are possible, with theprimary object being to block the circulating electrical currents fromthe electrodes. Some of these designs may help increase the mechanicalstrength of the measurement sensor loop or improve the manufacturabilityor other aspects of preferred embodiments of the invention. For example,one variation is a conduit having varying inner dimensions along thedirection of fluid flow through the shield. The shields may have arectangular cross-section such that the perimeter is reduced in amid-region along the direction of the fluid flow as illustrated in FIG.4, a rectangular cross-section such that the perimeter is enlarged in amid-region along the direction of the fluid flow as illustrated in FIG.5, a circular cross-section such that the circumference is reduced in amid-region along the direction of fluid flow as illustrated in FIG. 6,or a circular cross-section such that the circumference is enlarged in amid-region along the direction of fluid flow as illustrated in FIG. 7.Similar shields may be used having different cross-section shapes, suchas circular, square, triangular, elliptical, hexagonal, octagonal,irregular in shape, a combination of any of the above, or the likecross-section.

It should be noted that FIGS. 4-7 illustrate embodiments in which thecross-section of the ends (the entry and exit points of the fluid) arecongruent for illustrative purposes only, and that the cross-section ofthe ends of a shield may be of different sizes. For example, shields maybe used that have a tapered, increasing or decreasing, shape along thefluid flow. For example, FIG. 8 illustrates a rectangular-tapered shield800 having a rectangular cross-section such that the perimeter of areceiving end 810 is greater than the perimeter of an exiting end 812,and FIG. 9 illustrates a circular-tapered shield 900 having a circularcross-section such that the circumference of a receiving end 910 isgreater than the perimeter of an exiting end 912. It should be notedthat the receiving ends 810 and 910 are illustrated as being larger thanthe exiting ends 812 and 912, respectively, for illustrative purposesonly. Other embodiments of the present invention may orient the shieldssuch that the expected fluid flow enters the narrower end (e.g., ends812 and 912) and exits the larger end (e.g., ends 810 and 910).Furthermore, the thickness of the floor, sides and roof of each shieldmay be different from each other.

FIG. 10 illustrates a rectangular shield 1000 having point electrodes1008 in accordance with an embodiment of the present invention. Itshould be noted that the rectangular shield 1000 is illustrated ashaving transparent sides for illustrative purposes to better illustratethe placement of the point electrodes 1008. As illustrated in FIG. 10,the point electrodes are located along the inner side of opposingsidewalls 1010 and 1012. The point electrodes may be placed along thesidewalls 1010 and 1012, as illustrated in FIG. 10, or along the topwall 1014 and the bottom wall 1016. Additionally, the point electrodes1008 may be positioned in the middle of the shield, or closer to eitherend. However, it is preferred that the point electrodes be arranged suchthat a line drawn connecting the point electrodes 1008 is orthogonal tothe conductive fluid flow. It should be noted that the sensors (notshown) are preferably mounted along opposing interior walls of theshields as described below.

As other examples, FIGS. 11 and 12 illustrate medium-sized electrodes1108 and large-sized electrodes 1208, respectively, in accordance withan embodiment of the present invention. In these embodiments, the pointelectrodes 1008 of FIG. 10 are replaced with longitudinal electrodes,such as the medium-sized electrodes 1108 of FIG. 11 and the large-sizedelectrodes 1208 of FIG. 12. The position of the medium-sized electrodes1108 and the large-sized electrodes 1208 may be varied (e.g., closer toeither end of the conduit), but it is preferred that the medium-sizedelectrodes 1108 and the large-sized electrodes 1208 be arranged suchthat a line drawn connecting the electrodes is orthogonal to theconductive fluid flow.

FIG. 13 illustrates an example of large area electrodes that may be usedin accordance with the present invention. In particular, FIG. 13illustrates an example in which the electrodes cover a larger area, suchas the sides of shield 1300. In the embodiment illustrated in FIG. 13,electrodes 1308 substantially cover opposing sides 1310 and 1312 of theshield 1300. In this embodiment, it is preferred that the top 1314 andthe bottom 1316 be formed of a dielectric or insulating material. Thistype of electrode may have the advantage of the measured voltagedifference being independent of the fluid velocity distribution withinthe given shield, although this may not be a problem with a smallershield. A circular shield with point electrodes at the ends of theappropriate diameter provides a measured voltage that should indicate anaccurate fluid velocity regardless of whether any currents arecirculating within the shield, as long as the fluid flow is axiallysymmetric within the circular tube or column (again, currentscirculating within a small shield generally should not be a problem inpractice).

Alternatively, the electrodes may take on other shapes (e.g., curved,spiral, etc.) and other orientations (e.g., horizontal, diagonal, etc.),or combinations of the above. Preferably, each electrode is exposed tofluid flow on each side of the sidewall to which it is mounted.Alternatively, separate electrodes may be mounted on each side of agiven sidewall.

For various reasons, empirical calibration of the fluid flow velocitypreferably may be used, and may even be necessary.

It should be noted that some variation in the nominally regular spacingof the electrodes could be used to give greater mechanical strength tothe sensor loop. Also, some occasional variations in the overallthickness of the sensor loop shields may be used to enhance mechanicalstrength while maintaining a small overall size for running into and outof a well. Other means may be used for strengthening the sensor loop,such as a strong flexible insert above or below the shields, or a strongtop layer on top of the shields. Other possible ways to mechanicallystrengthen the sensor loop are possible and are considered to be withinthe spirit and scope of the present invention.

FIGS. 14-16 illustrate embodiments of the present invention in whichmultiple shields are used with multiple electrodes. In particular, FIG.14 illustrates the use of multiple rectangular shields to isolateadjacent groups (two or more) electrodes (not shown), and FIG. 15illustrates the use of multiple circular shields to isolate adjacentgroups of electrodes (not shown). FIG. 16 is similar to FIG. 15, exceptthat the space between the circular shields is filled with a dielectricor insulating material. This embodiment may have the added advantage ofproviding additional shielding and providing additional mechanicalsupport for the sensor ring.

These configurations may be used on a sensor loop as disclosed in thepreviously mentioned patent applications, such as patent applicationSer. No. 09/880,402, now U.S. Pat. No. 6,711,947. In particular, theconfiguration of FIG. 11 of the present disclosure may be used on radialsensing device 108 in FIGS. 2-4, 5D and 6 of patent application Ser. No.09/880,402, now U.S. Pat. No. 6,711,947. Furthermore, the configurationof FIG. 11 of the present disclosure may be used for electrodes 129 inFIGS. 8-9 and 10A-10B of patent application Ser. No. 09/880,402, nowU.S. Pat. No. 6,711,947. Again, both the shields and the electrodes maybe any of a wide variety of designs, some of which have been discussedabove.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. For example,different shield designs may be used within a single sensor loop, ordifferent electrode designs may be used within a single sensor loop.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

What is claimed is:
 1. A method of manufacturing a logging tool for aborehole, the method comprising: disposing pairs of voltage sensingelectrodes in a sensing device; forming shields interposed between eachof the pairs of voltage sensing electrodes, wherein the shields areconfigured to isolate each of the pairs of voltage sensing electrodesfrom ambient electrical current flow between the pairs of voltagesensing electrodes; coupling the sensing device to a tool bodyconfigured to be inserted into the borehole, wherein the sensing deviceis configured to detect radial flow of conductive fluid entering orleaving an interior wall of the borehole, and wherein the sensing deviceis configured to detect the conductive fluid flow adjacent the boreholeinterior wall.
 2. The method of claim 1, wherein the shields areinterconnected.
 3. The method of claim 1, wherein each of the shieldscomprises a conduit configured to allow fluid to flow therethrough. 4.The method of claim 1, wherein each of the voltage sensing electrodes isa point electrode.
 5. The method of claim 1, wherein each of the voltagesensing electrodes is a longitudinal electrode.
 6. The method of claim1, wherein each pair of electrodes substantially covers opposing sidesof the respective shields.
 7. The method of claim 1, wherein the sensingdevice is a sensor loop.
 8. The method of claim 1, further comprisingdisposing a magnetic field source in the sensing device proximate theelectrodes, wherein the magnetic field source is oriented to generate amagnetic field substantially orthogonal to an imaginary lineintersecting each pair of electrodes.
 9. The method of claim 8, whereinthe magnetic field source is an alternating magnetic field source. 10.The method of claim 8, wherein the disposing the magnetic field sourcefurther comprises: disposing a first coil of wire in the sensing deviceproximate the electrodes; and disposing a second coil of wire in thesensing device proximate the electrodes and offset from the first coilof wire.
 11. The method of claim 10, wherein the disposing the magneticfield source further comprises disposing a ferromagnetic materialbetween the first coil of wire and the second coil of wire.
 12. Themethod of claim 1, further comprising electrically coupling a respectiveresistor between each pair of electrodes.
 13. The method of claim 12,wherein the electrically coupling is performed before the disposing theelectrodes and the disposing the resistors.
 14. The method of claim 1,wherein the electrodes are regularly spaced in the sensing device. 15.The method of claim 14, wherein first and second electrodes in eachelectrode pair are spaced about 0.3 inches or less from each other. 16.The method of claim 1, wherein the sensing device has a substantiallyoval shape.